Various types of devices use optical fibers to generate, amplify, and/or transport high-power signals. For example, high-power pump lasers can be used to excite active ions in a core of an optical fiber and deposit energy, which is then extracted by signal beams. The energy transfer from pump to signal is naturally not perfect, and heat is generated as a consequence. Heat is a major performance limiter through mechanisms such as temperature-dependent refractive indices and mechanical stresses caused by temperature gradients. Both of these characteristics create various issues with laser-based devices.
With respect to temperature-dependent refractive indices, in high-power laser devices, a high beam quality is often of utmost importance. To obtain optimum beam quality, all laser power is ideally carried in a single mode, where the amplitude and phase are well-defined across a beam's cross-section so that the behavior of the beam can be manipulated with simple optical elements like lenses. Unfortunately, conventional devices used in the generation of high-power beams often allow more than one mode to exist. For example, Large Mode Area (LMA) fibers can guide more than one mode. Designers of high-power laser systems often take great care to avoid mode coupling, which occurs when a single-mode beam breaks up into several modes. A temperature-dependent refractive index is a major factor in creating mode coupling.
With respect to mechanical stresses, these stresses over the long-term can lead to mechanical failures. Before a mechanical failure occurs, however, mechanical stresses also cause lensing and stress-induced birefringence, which are additional mechanisms for causing mode coupling and therefore beam breakup.
As a result, the management of heat is often an important challenge for high-power laser devices. One conventional thermal management technique involves depositing a thin layer of metal over an optical fiber. The thickness of the metal layer could vary depending on the diameter of the optical fiber. For example, the thickness of the metal layer could vary between 5-60 microns. The metal layer is typically soldered onto another structure, such as a passive device having a large thermal mass. The metal layer transports heat away from the optical fiber to the large thermal mass.
Unfortunately, this approach can have various drawbacks. For example, the metal layer typically includes voids that limit heat transfer away from an optical fiber. Also, thicker metal layers may suffer from increased spalling stresses, which can result in device failures. Further, there are often fiber length limitations due to different coefficients of thermal expansion of the metal layer and a glass cladding of an optical fiber, and soldering at high temperatures can create thermal stresses in optical fibers. In general, this technique suffers from a number of limitations and secondary effects that can degrade device performance.